The present invention relates to the design of mixtures of sodium tripolyphosphate (STPP) for incorporation into detergents for use in automatic dishwashing machines, especially those designed for home use.
The detergent composition used in home automatic dishwashing machines is designed to remove dirt, food particles, and other organic materials so that the dishware will be visibly clean and effectively sterilized upon completion of the washing cycle of the machine.
A major component in many dishwashing detergents is an alkali metal polyphosphate, especially STPP. The selection of the proper polyphosphate for use in a formulation is dependent upon many factors, some of which are solubility, density, noncorrosive properties, sequestering and water-softening capacity, enhancement of cleaning function in the end use, as well as properties which assist the manufacture and stability of the final detergent product. STPP has been accepted by the formulators of dishwasher detergent mixtures as meeting many or all of the above requirements, as well as being quite economically beneficial to the formulation.
STPP is known in both the anhydrous and hexahydrate form. Two phases, or forms, of the anhydrous compound are known; Phase I is formed at high calcining temperatures, and Phase II is formed at lower temperatures. The solutions of Phase I and Phase II STPP will be identical, although the two phases differ in their hydration and dissolution properties. Phase I STPP will dissolve rapidly in water and will exhibit a high heat of hydration to form the hexahydrate. Phase II STPP will dissolve slowly in water and exhibits a lower heat of hydration. Generally speaking, Phase II STPP can be added to unagitated water without undue caking or lumping. Phase I STPP, when added to poorly agitated water, tends to lump or cake together as it forms a strongly supersaturated solution from which hexahydrate crystals are rapidly deposited. Commercial production has been directed towards producing mixtures of Phase I and Phase II STPP that will utilize these relative property differences for the improvement of manufacturing processes of laundry detergents. The mixtures of Phase I and Phase II STPP are prepared by variations in the relationship of calcining temperatures and time.
Detergent for automatic dishwashing machines is usually prepared by selecting a mixture of inorganic salts such as alkali metal polyphosphates, including tetrasodium pyrophosphate and STPP, and other salts such as sodium silicates, sodium sulfates, and sodium carbonates. To a mixture of the above, chlorine-containing materials, such as chlorinated trisodium phosphate or chloroisocyanurate salts, and surface active agents are added. Dyes and perfumes are added for product indentification and consumer appeal. The exact formulation will vary from manufacturer-to-manufacturer; however, the bulk density of the final product must be closely controlled since the automatic dispensers of automatic dishwashing machines are designed to operate and clean the dishware most efficiently when the detergent has a bulk density of at least 0.6 grams per cubic centimeter.
A method of manufacture of a dishwasher detergent is to add selected components in an aqueous solution or suspension to anhydrous granular STPP which is optionally mixed with other solid components such as chlorinated trisodium phosphate. Solutions of sodium silicate, a desirable component in the final detergent product, are added and serve as an agglomerating agent at this stage of the process. To prevent liquification or excessive lumping of the mixture, a portion of the free water present must be removed by reaction with STPP to form solid STPP hexahydrate. To enable this "dehydrating" reaction, the granular STPP particles must be readily penetrated by water from the aqueous mixture coating them or by some other means must be capable of allowing ready access of free water to anhydrous STPP.
Overheating of the formulation mixture, caused by heat of hydration of STPP, is to be avoided if chlorinated trisodium phosphate is present in the mixture. The material will melt in the vicinity of 50.degree. C., causing release of excessive free water and loss of active chlorine values.
Following the mixing step, the formulation is subjected to further processing steps which may include conditioning, drying, and sizing. Some free water may still be present in the final packaged product. Anhydrous STPP will need to be present and accessible to continue its functions as "dehydrating agent" by formation of STPP hexahydrate.
One object of this invention is to design a granular STPP product in which the Phase I/Phase II ratio in selected particle size fractions is such that the anhydrous STPP remains accessible for removal of free water by hydrate formation during the mixing step and, if necessary, on into the packaged product stage. Another object of this invention is to prepare a granular STPP product of a Phase I/Phase II ratio that will not cause overheating of the initial mixture in the formulation of automatic dishwasher detergent. An additional object of this invention is to provide the STPP product of an apparent density of 0.7 to 0.9 grams per cubic centimeter, found widely acceptable in the formulation of automatic dishwasher detergents.
While the above objectives may be applied to an STPP product made by several substantially different manufacturing methods, this invention is particularly applicable to STPP made by the method which employs a spray dryer as described below.